I was directed to MDN's  when it said, "for..in Iterates over the enumerable properties of an object."

Then I went to the  where it said "Enumerable properties are those which can be iterated by a for..in loop."

The dictionary defines enumerable as countable, but I can't really visualize what that means. Could i get an example of something being enumerable?

 

Well, whether a property is considered enumerable or not is based on its own . You can view this as part of the :

var descriptor = Object.getOwnPropertyDescriptor({ bar: 1 }, 'bar');console.log(descriptor.enumerable); // trueconsole.log(descriptor.value);      // 1console.log(descriptor);// { value: 1, writable: true, enumerable: true, configurable: true }

> var descriptor = Object.getOwnPropertyDescriptor({ bar: 1 }, 'bar');undefined>undefined> console.log(descriptor.enumerable); // truetrueundefined> console.log(descriptor.value);      // 11undefined>undefined> console.log(descriptor);{ value: 1, writable: true, enumerable: true, configurable: true }undefined> // { value: 1, writable: true, enumerable: true, configurable: true }undefined>

A for..in loop then iterates through the object's property names.

var foo = { bar: 1, baz: 2};for (var prop in foo)    console.log(prop); // outputs 'bar' and 'baz'

> var foo = { bar: 1, baz: 2};undefined>undefined> for (var prop in foo)...     console.log(prop); // outputs 'bar' and 'baz'barbazundefined

But, it only evaluates its statement -- console.log(prop); in this case -- for those properties whose [[Enumerable]] attribute is true.

This condition is in place because objects actually , especially :

> console.log(Object.getOwnPropertyNames(Object.prototype));[ 'constructor',  'toString',  'toLocaleString',  'valueOf',  'hasOwnProperty',  'isPrototypeOf',  'propertyIsEnumerable',  '__defineGetter__',  '__lookupGetter__',  '__defineSetter__',  '__lookupSetter__' ]undefined> // ["constructor", "toString", "toLocaleString", "valueOf", "hasOwnProperty","isPrototypeOf", "propertyIsEnumerable", /* etc. */]

console.log(Object.getOwnPropertyNames(Object.prototype));// ["constructor", "toString", "toLocaleString", "valueOf", "hasOwnProperty", "isPrototypeOf", "propertyIsEnumerable", /* etc. */]

Each of these properties still :

console.log('constructor' in foo); // trueconsole.log('toString' in foo);    // true// etc.

But, they're skipped (or "not counted") by the for..in loop because they're non-enumerable.

var descriptor = Object.getOwnPropertyDescriptor(Object.prototype, 'constructor');console.log(descriptor.enumerable); // false